1. Field of the Invention
This invention relates to a coupling device for coupling a photographing apparatus such as a single-lens reflex camera, a video camera, a broadcasting camera or the like with an accessory such as an interchangeable lens which is to be attached to and detached from the apparatus and more particularly to such a coupling device made of a ceramic material.
2. Description of the Prior Art
In a camera of the kind using an interchangeable lens, mounts which are disposed both on the side of a camera body and on the side of the lens are respectively provided with datum parts for ensuring correct positions of both of these mounts relative to an optical system so that the focal plane of the optical system can be set to coincide with a film surface. In mounting the lens on the camera, when these datum parts are set in their coinciding positions, the focal plane of the optical system of the interchangeable lens coincides with the film surface within the camera, so that a good, sharply focused photograph can be taken.
Even if lenses are frequently interchanged one with another, the relative positions of the mounts must be always correctly maintained and yet mounting and dismounting of these lenses must be always smoothly carried out.
To meet the above-stated requirement for durable precision, the mounts have been made of one of the materials prepared in the following manners:
(1) Brass-machining-nickel-chrome plating PA0 (2) Stainless steel-machining PA0 (3) Stainless steel-press-finishing and welding PA0 (4) A stainless steel alloy sintered body-machining PA0 (5) Aluminum alloy-machining-alumite treatment
The mounts have been thus made of a metal material such as brass, a stainless alloy steel, an aluminum alloy material, etc.
However, in cases where these mounts have been repeatedly subjected to strong abutting and sliding forces and external shocks which are exerted on them either singly or in combination during the mounting and dismounting operations repeated over a long period of time, the mounts on both sides wear or have scars or deformation. Then, the precision of their relative positions at the time of lens mounting decreases. In that event, it is hardly possible to obtain a sharp focus and the lens cannot be smoothly mounted and dismounted on and from the camera body. In the case of a mount made of a material which is prepared, for example, by plating brass with nickel-chrome or by applying an alumite treatment to an aluminum alloy, the nickel-chrome or alumite coating tends to peel off to accelerate thereby the degradation of precision and smoothness. Besides, the appearance impaired by the peeled off coating also degrades the value of the camera as a commercial article.
The conventional mount mechanisms for interchangeable lenses include a type called a spigot bayonet mount in which a bayonet mount member is arranged on the outside of one end of a lens barrel body to be rotatable relative to others as disclosed in U.S. Pat. No. 3,906,534; and another type called a total rotating mount which is arranged as disclosed in U.S. Pat. No. 4,247,190 and U.S. Pat. No. 4,230,403. In the case of these U.S. patents, the mount mechanism is made of a metal material.
In the case of metal mount members, the internal strain of the metal material or an internal strain which is caused by machining work or is developed under an assembled state brings about a secular change in dimensions after the lapse of a long period even without repeating mounting and dismounting operations. The metal mount member thus has presented another problem in terms of degradation in precision. Furthermore, to aid mounting and dismounting smoothness, sometimes has been applied a lubricant or the surface of the member has been hardened by nitriding, hard chrome plating or by applying a hard alumite treatment. However, this either impairs dimensional precision or increases the cost due to an additional manufacturing process. An improvement has been desired also in terms of performance.
It is an object of the present invention to provide a coupling device which is capable of preventing degradation of precision due to flaws such as scars and wear and external flaws such as scars and rust, which have been the drawbacks of the conventional coupling devices made of the above-stated materials.
It is another object of this invention to provide a coupling device which is divided into a plurality of bodies and in which the above-stated problems are solved by using a ceramic material for a part arranged to come into sliding contact with a mount member of a body to be coupled therewith.